Stair Trim Molding: Finish Suggestions for a Custom-made Staircase

 I need some advice on stair trim molding. I have a short stairway that goes from our mud room to our kitchen (seven treads) that our contractor built during a remodel last year using some pretty simple wood treads. Unfortunately, he didn’t put in any sort of trim molding on the stairs at the time, so the joint between the wall and the stair doesn’t look very good. We had done a ton of work during this remodel and been out of the house for months, so we were ready to get the contractor out and had bigger issues than trying to get him to fix this. 

Now the time has finally come for me to finish these stairs but I can’t find any information on how to install stair trim molding. I’m open to pulling out the treads and risers, installing the molding and then putting in better, nicer treads to replace the existing ones, but I’d like to know what my options are and if there’s a way to add stair trim molding without tearing up the staircase.

The Money Pit Answer

The reason you can't find trim molding to fit this situation is that there is none! Your contractor hand-built these stairs, which is commendable, but he did so in such an unusual way that there is no molding designed to fit the space.
Typically, this type of stairwell would call for a box stair in which treads and risers are routed into side pieces called stringers. Stringers provide structural support and can be trimmed between the staircase and the wall, typically with quarter-round or shoe molding. 
In this case, it looks like the finish work was well done with little or no gap between the treads/risers and the walls. So, my recommendation would be for you to do nothing with this structurally. I'd suggest you caulk any gaps between the treads and the walls and then carefully paint right up to the edge of the wood. If you would like to have the stair look a bit more normal you could also consider painting a box beam on the wall where it normally would be.  By doing so you could paint this in a faux finish and use a gloss paint that would also be easy to clean when scuff marks develop.
In one of my early jobs in home improvement, I used to build stairs and railings and found that if I made any goofs, I'd make the repair look so good that everyone thought it was supposed to look that way in the first place. I think if you took a shot at painting on a box beam, you'll solve your stair trim molding problem and draw a few compliments on your creativity in the process!